1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving control channels in a wireless communication system, and more particularly, to an apparatus and method for transmitting/receiving control channels on the uplink in a mobile communication system
2. Description of the Related Art
Generally, mobile communication systems have been developed to support communication while guaranteeing mobility for users. Due to the rapid progress of the communication technologies, such mobile communication systems are developing into advanced communication systems capable of supporting not only the voice communication, but also the high-speed data communication. Now, the mobile communication system has evolved to support higher-speed data communication, an example of which is an Enhanced Universal Terrestrial Radio Access (EUTRA) system, which is the next generation mobile communication standard proposed by 3rd Generation Partnership Project (3GPP).
Mobile communication systems can be classified into various types, such as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA). Among them, CDMA has been generally used. However, since CDMA has difficulty in supporting a large volume of data at high speed due to the limited number of orthogonal codes, Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-Frequency Division Multiple Access (SC-FDMA), which are a kind of FDMA, are now applied as downlink and uplink standard technologies of EUTRA, respectively.
In the EUTRA system, uplink control information includes Acknowledgement (ACK)/Negative ACK (NACK) feedback information, which is a signal used for feeding back success/failure in reception of downlink transmission data. The uplink control information also includes Channel Quality Indication (CQI) information used for feeding back downlink channel quality. The ACK/NACK information, which is generally composed of 1 bit, is repeatedly transmitted several times for improvement of its reception performance and expansion of the cell coverage. ACK/NACK is defined herein as ACK or NACK.
Generally, the CQI information is composed of multiple bits to express the channel quality, and is transmitted after undergoing channel coding for reception performance improvement and cell coverage expansion. Block coding or convolutional coding is available as the channel coding for the CQI information. A reception reliability required in receiving the control information is determined according to the type of the control information. Generally, ACK/NACK, which requires a Bit Error Rate (BER) of about 10−2˜10−4, is lower in required BER than CQI, which requires a BER of 10−2˜10−1.
In the EUTRA system, when a User Equipment (UE), or a mobile station, transmits only the uplink control information channel without data, it uses an allocated particular frequency band for control information transmission. A physical channel for transmitting only the control information is defined as Physical Uplink Control Channel (PUCCH), and the PUCCH is mapped to the allocated particular frequency band.
FIG. 1 is a diagram illustrating a transmission structure for a physical channel PUCCH for transmitting control information on the uplink in a 3GPP EUTRA system.
In FIG. 1, the vertical axis represents a frequency domain and the horizontal axis represents a time domain. A scope of the time domain is shown as one subframe 102, while a scope of the frequency domain is shown as a system transmission bandwidth 110. The subframe 102, which is a basic transmission unit of the uplink, has a 1-ms length, and one subframe is composed of two 0.5-ms slots 104 and 106. Slots 104 and 106 are each composed of multiple SC-FDMA symbols 111˜124, and 131˜144, respectively. FIG. 1 illustrates an example where one slot is composed of 7 SC-FDMA symbols.
The minimum unit of the frequency domain is a subcarrier, and the basic unit of resource allocation is a Resource Block (RB), 108 and 109. The RBs 108 and 109 are composed of multiple subcarriers and multiple SC-FDMA symbols. In the example shown in FIG. 1, 12 subcarriers and 14 SC-FDMA symbols constituting 2 slots form one RB. Even in the downlink to which OFDM transmission is applied, one RB is generated from 12 subcarriers and 14 OFDM symbols.
Frequency bands, to which the PUCCHs are mapped, correspond to RBs 108 and 109 situated on both ends of the system transmission bandwidth 110. The PUCCH can undergo frequency hopping to increase its frequency diversity during one subframe, and in this case, slot-by-slot hopping is possible. An Evolved Node B (ENB), or base station, can occasionally allocate multiple RBs for PUCCH transmission to approve transmission of control information from multiple users. In FIG. 1, the frequency hopping is shown by reference numeral 150 and reference numeral 160. A detailed description of the frequency hopping is provided below.
Control information #1, which was transmitted through the pre-allocated RB 108 in the first slot 104, is transmitted through another pre-allocated RB 109 after undergoing frequency hopping in the second slot 106. On the contrary, control information #2, which was transmitted through the pre-allocated RB 109 in the first slot 104, is transmitted through another pre-allocated RB 108 after undergoing frequency hopping in the second slot 106.
In the example of FIG. 1, in one subframe 102, the control information #1 is transmitted on SC-FDMA symbols 111, 113, 114, 115, 117, 138, 140, 141, 142 and 144, and the control information #2 is transmitted on SC-FDMA symbols 131, 133, 134, 135, 137, 118, 120, 121, 122 and 124. Reference Signals (RSs), also known as pilots, are transmitted in Pilot SC-FDMA symbols 112, 116, 139 and 143 (or 132, 136, 119 and 123). The RS is generated with a predetermined sequence, and used for channel estimation for coherent demodulation at a receiver. In FIG. 1, the number of SC-FDMA symbols for control information transmission, the number of SC-FDMA symbols for RS transmission, and their positions in the subframe are shown by way of example, and these are subject to change according to the types of desired transmission control information and/or the system operation.
Code Division Multiplexing (CDM) can be used to multiplex uplink control information for different users, such as ACK/NACK information, CQI information, and Multiple Input Multiple Output (MIMO) feedback information transmitted over PUCCH. CDM is robust against interference signals compared with Frequency Division Multiplexing (FDM).
A Zadoff-Chu (ZC) sequence is under discussion as a sequence to be used for CDM-multiplexing the control information. Since the ZC sequence has a constant signal amplitude in the time and frequency domains, it has a good Peak-to-Average Power Ratio (PAPR) characteristic and shows excellent channel estimation performance in the frequency domain. Further, the ZC sequence has a characteristic that a circular autocorrelation for a non-zero shift is zero (0). Therefore, UEs transmitting control information using the same ZC sequence can be identified using different cyclic shift values of the ZC sequence.
In the actual wireless channel environment, different cyclic shift values are set for the users intending to undergo multiplexing so as to satisfy a condition that they are greater than the maximum transmission delay of the wireless transmission path, thereby maintaining inter-user orthogonality. Therefore, the number of users capable of multiple access is determined from the length and cyclic shift values of the ZC sequence. The ZC sequence can be applied even to the SC-FDMA symbols for RS transmission, to identify RSs of different UEs using the cyclic shift values.
Generally, a length of the ZC sequence used for the PUCCH is assumed to be 12 samples, which is equal to the number of subcarriers constituting one RB. In this case, since the maximum possible number of different cyclic shift values of the ZC sequence is 12, it is possible to multiplex a maximum of 12 PUCCHs to one RB by allocating different cyclic shift values to the PUCCHs. The Typical Urban (TU) model, which is the wireless channel model generally considered in the EUTRA system, applies cyclic shift values to PUCCHs at intervals of at least 2 samples due to the frequency-selective channel characteristic. Applying the cyclic shift values at intervals of at least 2 samples in this way restricts the number of cyclic shift values within one RB to 6 or less. In this manner, the orthogonality between PUCCHs that are mapped to cyclic shifts on a one-to-one basis can be maintained without its abrupt loss.
FIG. 2 illustrates an example of multiplexing CQIs for users with different cyclic shift values of the ZC sequence within the same RB when transmitting CQIs over the PUCCHs having the structure of FIG. 1.
In FIG. 2, the vertical axis represents cyclic shift values 200 of a ZC sequence. In the TU model assumed as the wireless channel, since the maximum number of channels that can undergo multiplexing without an abrupt orthogonality loss within one RB is 6, there is shown an occasion where 6 CQIs 202, 204, 206, 208, 210 and 212 undergo multiplexing. In the example of FIG. 2, the same RB and the same ZC sequence are used for transmission of the CQI information in such a manner that CQI 202 from UE #1 is transmitted using a cyclic shift ‘0’ 214; CQI 204 from UE #2 is transmitted using a cyclic shift ‘2’ 218; CQI 206 from UE #3 is transmitted using a cyclic shift ‘4’ 222; CQI 208 from UE #4 is transmitted using a cyclic shift ‘6’ 226; CQI 210 from UE #5 is transmitted using a cyclic shift ‘8’ 230; and CQI 212 from UE #6 is transmitted using a cyclic shift ‘10’ 234. With reference to FIG. 1, a description will be made as to how the control information signal is mapped to the ZC sequence during CDM transmission of the control information based on the ZC sequence. A length-N ZC sequence for UE #i is defined as g(n+Δi) mod N, where n=0, . . . , N−1, Δi denotes a cyclic shift value for UE #i, and i denotes a UE index used for identifying a UE. Also, a control information signal that a UE #i intends to transmit is defined as mi,k, where k=1, . . . , Nsym. If Nsym denotes the number of SC-FDMA symbols for control information transmission within one subframe, a signal ci,k,n (nth sample of a kth SC-FDMA symbol of UE #i) mapped to each SC-FDMA symbol is defined as Equation (1).ci,k,n=g(n+Δi)mod N×mi,k  (1)where k=1, . . . , Nsym, n=0, 1, . . . , N−1, and Δi denotes a cyclic shift value of a ZC sequence for a UE #i.
In the example of FIG. 1, in one subframe, the number Nsym of SC-FDMA symbols for control information transmission is 10, excluding the 4 SC-FDMA symbols for RS transmission, and a length N of the ZC sequence is 12, which is equal to the number of subcarriers constituting one RB. From the standpoint of one UE, a cyclic-shifted ZC sequence is applied to each SC-FDMA symbol, and its desired transmission control information signal is generated in such a manner that one modulation symbol is multiplied by the ZC sequence cyclic-shifted in the time domain at every SC-FDMA symbol for control information transmission. Therefore, a maximum of Nsym control information modulation symbols can be transmitted per subframe. That is, in the example of FIG. 1, a maximum of 10 control information modulation symbols can be transmitted during one subframe.
It is possible to increase the multiplexing capacity of PUCCHs for transmitting the control information by additionally applying time-domain orthogonal covers, aside from the CDM control information transmission based on the ZC sequence. A Walsh sequence can be an example of the orthogonal cover. There are M orthogonal sequences as length-M orthogonal covers. Specifically, for the 1-bit control information like ACK/NACK, its multiplexing capacity can be increased by applying time-domain orthogonal covers to the SC-FDMA symbols to which ACK/NACK is mapped during transmission. The EUTRA system considers that the PUCCH for ACK/NACK transmission uses 3 SC-FDMA symbols for RS transmission per slot for improvement of channel estimation performance. Therefore, in the example of FIG. 1 where one slot is composed of 7 SC-FDMA symbols, 4 SC-FDMA symbols for ACK/NACK transmission are available. The orthogonality loss caused by a change in wireless channels can be minimized by restricting the time interval, where the time-domain orthogonal covers are applied, to one slot or less. Length-4 orthogonal covers are applied to the 4 SC-FDMA symbols for ACK/NACK transmission, and length-3 orthogonal covers are applied to the 3 SC-FDMA symbols for RS transmission.
Regarding the ACK/NACK and RS, their users can be identified with cyclic shift values of the ZC sequence, and the additional user identification is possible by the orthogonal covers. For coherent reception of ACK/NACK, since there should exist RSs that are mapped to ACK/NACK on a one-to-one basis, the multiplexing capacity of ACK/NACK signals is restricted by the RSs mapped to ACK/NACK. For example, in the TU channel model where 6 cyclic shift values are considered per RB, since different length-3 time-domain orthogonal covers can be applied to cyclic shifts of the ZC sequence, which are applied to RSs, RSs from a maximum of 18 different users can undergo multiplexing. Regarding ACK/NACK, since it is mapped to RSs on a one-to-one basis, a maximum of 18 ACK/NACK signals can be multiplexed per RB. In this case, there are 4 length-4 orthogonal covers applied to ACK/NACK, and among them, 3 orthogonal covers are used. The orthogonal covers applied to ACK/NACK can be recognized in common between a UE and an ENB under agreement previously made therebetween, or by signaling. As a result, it is possible to increase the multiplexing capacity three times compared with the case where the time-domain orthogonal covers are unused.
FIG. 3 illustrates an example of multiplexing ACK/NACK for each user with different cyclic shift values of the ZC sequence and additional time-domain orthogonal covers in the same RB in the PUCCH structure for ACK/NACK transmission.
In FIG. 3, the vertical axis represents cyclic shift values 300 of a ZC sequence, and the horizontal axis represents time-domain orthogonal covers 302. In the TU model assumed as the wireless channel, if the maximum number of cyclic shifts that can undergo multiplexing without an abrupt orthogonality loss within one RB is 6, and 3 length-4 orthogonal covers 364, 366 and 368 are additionally used, a maximum of 18 (=6*3) ACK/NACK signals 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330, 332, 334, 336 and 338 can undergo multiplexing. In the example of FIG. 3, the same RB and the same ZC sequence are used for ACK/NACK transmission in such a manner that ACK/NACK 304 from UE #1 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal cover ‘0’ 364; ACK/NACK 306 from UE #2 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal cover ‘1’ 366; ACK/NACK 308 from UE #3 is transmitted using a cyclic shift ‘0’ 340 and an orthogonal cover ‘2’ 368; . . . ; ACK/NACK 334 from UE #16 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal cover ‘0’ 364; ACK/NACK 336 from UE #17 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal cover ‘1’ 366; and ACK/NACK 338 from UE #18 is transmitted using a cyclic shift ‘10’ 360 and an orthogonal cover ‘2’ 368. The orthogonal covers 364, 366 and 368 are length-4 orthogonal codes satisfying orthogonality therebetween.
In transmitting the CQI or ACK/NACK through the PUCCH in this way, there is a possible case where one UE should simultaneously transmit CQI and ACK/NACK within one subframe. In its most typical case, several subframes before it transmits CQI, a UE receives its scheduled downlink data channel from an ENB over a downlink control channel. Upon receipt of the downlink control channel, the UE receives the data from the RB where the downlink data is transmitted, decodes the received data, and transmits ACK/NACK corresponding to the decoding result. If the subframe where the UE should transmit the ACK/NACK is coincident in timing with the subframe where it should transmit CQI, the UE must transmit the ACK/NACK and CQI together within the subframe. The UE should transmit RS even when transmitting the ACK/NACK and CQI. However, since such matters have not been discussed yet in the standard, there is a need for a transmission/reception apparatus and method for simultaneous transmission of ACK/NACK and CQI, and simultaneous transmission of RS.
In this case, even when a UE has failed in reception of the scheduling control channel transmitted over the downlink, the UE can transmit only the CQI channel. Then, though the ENB waits for reception of ACK/NACK information for the scheduling control channel, the UE transmits CQI information in reality. In this case, even though the UE has transmitted only the CQI information, the ENB may misrecognize that it has received ACK/NACK information and CQI information from the UE. Further, the ENB may detect the nonexistent ACK/NACK information from the CQI channel, generating an error. Therefore, the ENB may misrecognize the control information transmitted from the UE, probably causing a communication failure.